This invention pertains to gallium-containing zeolite catalysts, particularly to gallium-containing zeolite catalysts which are useful in aromatization of light paraffins, and in other hydrocarbon conversion reactions.
Zeolitic materials, both natural and synthetic, are known to have catalytic capabilities for various types of hydrocarbon conversions. Certain zeolitic materials are ordered, porous, crystalline aluminosilicates having a definite crystalline structure within which there are a large number of small cavities which are interconnected by a number of still smaller channels. These cavities and channels are precisely uniform in size. Because the dimensions of these pores will admit molecules of certain dimensions, while rejecting those of larger dimensions, these materials have come to be known as "molecular sieves," and are used in a variety of ways to take advantage of these properties.
U.S. Pat. No. 3,702,886, which is incorporated by reference, describes a family of synthetic zeolites designated as "Zeolite ZSM-5" or simply "ZSM-5," having a characteristic x-ray diffraction patterns and a composition in terms of mole ratios of oxides as follows: EQU 0.9.+-.0.2M.sub.2/n O:W.sub.2 O.sub.3 :5-100 YO.sub.2 :zH.sub.2 O
wherein M is a cation, n is the valence of said cation, W is selected from the group consisting of aluminum and gallium, preferably aluminum; Y is selected from the group consisting of silicon and germanium, preferably silicon; and z is between 0 and 40. Crystalline aluminosilicates having the aforesaid composition and characteristic X-ray diffraction pattern are classified as MFI-type zeolites in the Atlas of Zeolite Structure Types by W. M. Meier and D. H. Olson, published by the Structure Commission of the International Zeolite Association (1978), which is incorporated by reference. U.S. Pat. No. 3,702,886 was the forerunner to a number of patents relating to synthetic crystalline aluminosilicate zeolites, all of which are characterized by a high, that is 10:1 or greater, silica to alumina molar ratio, high stability, presence of acid sites, and the ability to catalyze many kinds of conversion reactions, such as cracking, isomerization of n-paraffins and napthenes, polymerization of olefinic and acetylenic compounds, reforming, alkylation, isomerization of polyalkyl substituted aromatics, and disproportionation of aliphatic and alkyl substituted aromatic hydrocarbons. The acid form of a ZSM zeolite may be denoted as an "HZSM."
Gallium-containing zeolites for catalysis, particularly for catalysis of light paraffin aromatization, have received attention recently. Different formulations of such catalysts have included catalysts prepared by ion exchange, by impregnation of ZSM-5 with gallium salts, and by synthesis of galloaluminosilicates.
Gnep et al., "Conversion of Light Alkanes into Aromatic Hydrocarbons. 3. Aromatization of Propane and Propene on Mixtures of HZSM5 and of Ga.sub.2 O.sub.3," in Karge et al. (Ed.), Zeolites as Catalysts, Sorbents and Detergent Builders, pp 153-162 (1989) report the use of a mechanical mixture of Ga.sub.2 O.sub.3 and HZSM-5 as a catalyst for the conversion of propane, propene, 1-hexene, 1-heptene, methylcyclohexane and methylcyclohexene. This reference does not discuss the degree of mechanical mixing used, and in particular does not mention any milling of the catalyst; does not mention any reduction or hydrogen pretreatment of the catalyst; and does not mention any time-on-stream increase in catalytic activity. This reference required relatively high weight ratios of gallium to ZSM-5 to achieve modest aromatics selectivity. In FIG. 1 of Gnep et al., the lowest elemental weight percentage of gallium in the catalyst (corresponding to 5 mg Ga.sub.2 O.sub.3 /25 mg HZSM5) may be calculated to be approximately 12.4%, which was reported to give an aromatics yield of less than 1/4% at a total propane conversion of about 5%, or an aromatics selectivity of less than 5% (the ratio of aromatics yield to total conversion). In the same FIG. 1 of Gnep et al., where the elemental weight percentage of gallium was approximately 55.8% (corresponding to 75 mg Ga.sub.2 O.sub.3 /25 mg HZSM5), the aromatics yield was reported to be about 11/2% at a total propane conversion of about 83/4%, or an aromatics selectivity of about 17%. In FIG. 3 of Gnep et al., for the same approximately 55.8 wt % gallium catalyst, at a total reported propane conversion of about 23.5%, the reported aromatics yield was about 10%, or an aromatics selectivity of about 42.5%. In the same FIG. 3 of Gnep et al., for the approximately 12.4 wt % gallium catalyst, at a total reported propane conversion of about 11.3%, the reported aromatics yield was about 1.6%, or an aromatics selectivity of about 14%.
U.S. Pat. No. 4,642,403 describes the preparation of gallium/zeolite catalysts by impregnation or ion-exchange. It discusses activation of the catalyst by heating from 400.degree. C. to 650.degree. C., preferably from 500.degree. C. to 600.degree. C. It states that activation may be carried out in an atmosphere consisting of hydrogen, air, steam, or a gas inert under the reaction conditions such as nitrogen, but preferably in an atmosphere containing oxygen. It describes in-situ hydrogen treatment for 2 hours at 550.degree. C. prior to testing for hydrocarbon conversion activity.
U.S. Pat. No. 4,766,265, Catalysts for the Conversion of Ethane to Liquid Aromatic Hydrocarbons, mentions treating HZSM-5 with a source of gallium, aluminum, and/or zinc, by ion exchange, impregnation, gas phase displacement, or other known methods of incorporating the metal into the molecular sieve. The preferable metal is stated to be gallium, and the reference describes incorporating gallium by impregnation or ion-exchange with a gallium-containing solution. This reference teaches the desirability of additionally further treating the gallium-loaded zeolite both with rhenium, and with a metal selected from the group consisting of nickel, palladium, platinum, rhodium, and iridium. This reference describes conversions of ethane, so its reported experimental results are not directly comparable to those described below for conversion of propane. With that caveat, a total paraffin conversion in Table I of 48.3% was reported, obtained with a Ga/Re/Rh/HZSM-5 catalyst at 640.degree. C., WHSV=0.73, with an aromatics selectivity of 60.0%; but with a high 27.1% selectivity for methane, an undesired product. Table I of this reference reports catalysts with higher aromatics selectivities, but at lower total paraffin conversions. Table I of this reference also reports catalysts with lower methane selectivities, but at lower total conversions, lower aromatics selectivities or both. This reference discusses no pretreatment to activate the catalysts, other than heating in nitrogen to the reaction temperature.
U.S. Pat. No. 4,761,511 teaches the wet preparation of certain crystalline galloaluminosilicates, and states that steam-modified galloaluminosilicates showed increased catalysis of hydrocarbon aromatization. Aromatics selectivities up to 57.7% were reported for butane conversion, and up to 55% for propane conversion.